Implantable tissue stimulators such as implantable pacemakers and implantable drug injection systems conventionally use an implanted battery as a power source. Although typical pacemaker batteries have a relatively long life, they do eventually require replacement. Such a replacement requires removal of the pacemaker and its subsequent reimplantation, thereby causing the patient to incur an additional, although minimal, risk. Consequently there has been a need to determine life remaining in an implanted battery so that it can be replaced at the optimal time, early replacement subjecting the patient to unnecessary risk and late replacement subjecting him to a possibility that his implanted device may fail. One technique for determining remaining battery life is described in U.S. Patent Application Ser. No. 16,200 filed Feb. 28, 1978. Basically, this technique loads the battery with a predetermined load and monitors the output voltage, this voltage indicating the internal impedance of the battery. Although this system has been generally satisfactory, it is independent upon the value of the loading impedance remaining known and constant. The present invention eliminates a need for loading the battery with predetermined load impedances by providing a closed loop servo means which depends for accuracy upon a reference voltage having a known value.